Dimerization of olefins is well known and industrially useful. In particular, dimerization of 2-methylpropene to produce 2,4,4-trimethylpentene, commonly called isooctane, is a well known and useful reaction because the product can be used for gasoline reformulation. Branched saturated hydrocarbons, such as isooctane, have a high octane number, low volatility and do not contain sulfur or aromatics, and are, therefore, particularly useful for improving gasoline and making it more environmentally friendly. Dimerization of linear olefins also represents an attractive route for the production of high octane number blending components. The branched species, however, may also contribute to engine deposits. Thus, in some instances the lower octane number of products of dimerization of linear olefins may be offset by lower engine deposits.
Branched saturated hydrocarbons can be produced in different ways, e.g. by alkylation of olefins with isoparaffins and by dimerization of light olefins, in some instances followed by hydrogenation. Alkylation of 2-methylpropene (isobutene) with isobutane directly produces isooctane, and the dimerization reaction of 2-methylpropene produces 2,4,4-trimethyl-1-pentene and 2,4,4-trimethyl-2-pentene, amongst other products.
Use of ionic liquids for dimerization and/or oligomerization of olefins is well known, including for example, the processes disclosed in U.S. Pat. Nos. 5,304,615; 5,731,101; 6,706,936; 6,518,473 and 7,351,780, the disclosures of which are incorporated herein by reference.
Ionic liquids make an ideal solvent because they have very low volatility, and do not evaporate or burn very easily, resulting in safer processes. Also, the low melting point and negligible vapor pressure lead to a wide liquid range often exceeding 100 degrees C. (unlike the hundred degree Celsius range limit found for liquid water). Another advantage is that chemical and physical properties of ionic liquids can be “tuned” by selecting different anion and cation combinations, and different ionic liquids can be mixed together to make binary or ternary ionic liquids. It is even possible to have ionic liquid solvents that also function as catalysts or co-catalysts in reactions.
Perhaps the most important benefit of using ionic liquids in various reactions is simplified separation of the products. Most ionic liquids are polar, and hence non-polar products—like isooctane and octane—are immiscible therein. The biphasic process allows separation of the products by decantation and reuse of the catalysts. Further, the fact that the product is not miscible in the solvent, also tends to drive the reaction.
It is known that catalysts may be either heterogeneous or homogenous catalysts. Homogeneous catalysts act by reacting within a single phase, and the catalyst is not supported. While a reaction involving a homogenous catalyst may result in a narrow weight distribution of products, it may be unfavorable because it can be difficult to separate the product from the reactants.
Heterogenous catalysts act by reacting at the boundary of two phases (such as a solid-liquid phase). While heterogeneous catalysts may be less selective than homogenous catalysts, they are advantageous in that the products are easier to separate and can offer a continuous manufacturing process. It would be advantageous therefore, to have a catalyst that provides the separability of a heterogenous catalyst with the ease of synthesis that can be achieved with ionic liquids.
It would be desirable, therefore, to utilize ionic liquids to attach oligomerization catalysts onto a solid support, thus providing an oligomerization catalyst system and reaction process useful in fixed bed reactors and which is readily used in a refinery environment.